1. Field of the Invention
This invention relates to a method for producing a diffraction grating in optical elements with a light-propagating optical waveguide. The diffraction grating is used to control waveguide light in the optical waveguide or to optically link the inside and outside of the optical waveguide.
2. Description of the Prior Art
In recent years, various optical elements for propagating light in an optical waveguide formed from a nonlinear optical material have been developed. Since an optical waveguide has very little light loss, even though its cross section is only about as large as the wavelength of the light to be propagated, there is none of the light spread which accompanies the propagation of light, so that the conversion of wavelength can be attained over the entire coherence length, the conversion efficiency can be increased, and other advantages can be realized. For example, optical waveguides are particularly useful for generating second harmonic light with a wavelength half that of the fundamental mode light by the use of second harmonic generation from nonlinear optical materials. In order to generate second harmonic light, it is important to confine the fundamental mode light in as narrow an area as possible with low loss, because the efficiency of converting from the fundamental mode light to the second harmonic light is proportional to the square of the optical density of the fundamental mode light. Therefore, a wavelength conversion element in which the optical waveguide is formed from a nonlinear optical material can confine the fundamental mode light in the optical waveguide to generate second harmonic light, thereby attaining a conversion of wavelength with high efficiency.
However, in the case of optical wavelength conversion elements, there are disadvantages that degrade the conversion efficiency of wavelength, such as difficult entry of the fundamental mode light into the optical waveguide due to its extremely small size, poor crystallinity of the nonlinear optical material caused by the formation of the optical waveguide, and, in particular, scattering of the fundamental mode light due to unevenness on the optical waveguide surface.
Previously, directing the fundamental mode light into the optical waveguide was accomplished, as shown in FIG. 5, by optically polishing the end 32a of an optical waveguide 32, which is composed of nonlinear optical crystal material and formed into a substrate 31, and condensing, by means of a lens 50 with a large numerical aperture (NA), a fundamental mode light emitted, for example, from a semiconductor laser 40. The fundamental mode light 21 propagated in the optical waveguide 32 becomes second harmonic light due, for example, to Cerenkov radiation. However, in order to direct the fundamental mode light into the optical waveguide 32, the optical axis must be adjusted precisely between the extremely small end 32a of the optical waveguide 32 and the lens 50.
For this reason, instead of using a lens 50, a diffraction grating optical coupler is used to direct the fundamental mode light into the optical waveguide 32. A diffraction grating optical coupler is a diffraction grating on the optical waveguide for directing light into the optical waveguide, and because it does not require a lens or the like, the wavelength conversion device can be made more compact and the adjustment of the optical axis between the lens and the optical waveguide is not required.
In order to obtain high coupling efficiency in a diffraction grating optical coupler, the lattice pitch of the diffraction grating must be extremely fine. Furthermore, in order to make wavelength conversion devices and similar devices using an optical element more compact, it is desirable to make the diffraction grating optical coupler a wave front conversion type accompanied by a lens or other functional elements. Therefore, it is desirable to use an electron beam exposure technique with high resolution and high-speed deflectivity.
However, when forming a diffraction grating on an optical waveguide by an electron beam exposure technique, the resist film formed on the substrate of nonlinear optical material becomes charged when irradiated with an electron beam since the nonlinear optical material is usually an insulator, thus causing the electron beam path to bend. This makes it impossible to draw a fine pattern on the optical waveguide with an electron beam. Although the deposition of a metal thin film on the optical waveguide is considered useful for preventing the resist film from being charged, a process for removing the metal thin film would be required and the agents used in removal may attack the optical waveguide.